This article from our guest expert, Mehedi Rashid, discusses the design for design for wind and/or seismic resistant structures, the response modification factor, and the strong column weak beam designs.
A typical question that comes up more than often is, “If the loads of my design analysis are controlled by wind loading over seismic, should I still consider seismic design?”
The answer to the question above is, “yes” if the structure will be constructed in an area with moderate to high seismic risk. Although quite frequently, design wind loading may exceed the seismic design base shear, an insight behind the seismic design philosophy may provide the basis for the said answer.
But before I get into the seismic design philosophy it is important to mention that a typical structure is usually designed to remain elastic during a design wind event. Design to remain elastic means that the load deformation curve of a structure can be represented by a straight line, as shown in Figure-1.
Designing For Wind Resistant Structures
Structure remain elastic also means that none of the structural elements within the structure has experienced yielding or has formed plastic hinges. Once the structure elements start to yield or form plastic hinges, the load deformation curve deviate from the straight line and follow a more non-linear load deformation characteristic. The structure is expected to experience varying level of damages during this stage.
For seismic design however, the structure is designed to behave differently compared to the wind design approach. In a high seismic area, when a design earthquake hits a very stiff non deformable structure, the structure can experience a very large lateral force caused by the inertia of the building. This force in many instances can be several times the force that can be generated by the wind loading.
Designing for Seismic Resistant Structures
However, in most instances, a given structure designed for seismic resistance deforms dissipating some of the seismic energy causing the high inertia force to reduce. In structural engineering, a structure that is not designed to deform and sustain damage during an earthquake is termed as design for elastic seismic base shear. Therefore, more a structure can deform past its elastic limit during an earthquake and sustain some damage, lesser the force that it will see for design.
Design for a very large force can render a structure uneconomical to construct as more material and labor will be needed to construct the structure. This is why a structure should be allowed to deform and experience some damage without complete collapse during a large earthquake to reduce the seismic design forces and therefore reduce construction cost and make the structure more constructible. All modern seismic resistant design codes recognize this and allow deformation and damage to the structure during a design earthquake.
Response Modification Factor
The ratio of the seismic force that a structure can really experience without any apparent deformation (i.e. remain elastic) to the seismic force that a structure is really designed for is called a Response Modification Factor, R. Therefore, a structure with high Response Modification Factor (R) can be designed for a lower earthquake force but will need to be provided with very high deformation capability as shown in Figure-2.
What is represented in Figure-2 is based on equal displacement demand theory. This is the basis for force based design philosophy of modern earthquake resistant structures. Equal displacement demand means that a structure with R greater than 1 will need to be provided with the same displacement demand as of a structure that is designed to remain fully elastic (i.e. R = 1) during a seismic event. This figure also indicates that as the Response Modification Factor, R is increased the corresponding deformation demand past the elastic range is also increased.
Strong Column Weak Beam Designs
In a multi-story building structure, usually large deformation is mainly provided by the rotation of plastic hinges that form in the beams of a beam column moment resisting frame system. A typical pattern of formation of plastic hinges is shown in the Figure – 3 below.
In order to maintain stability of the structure, the columns of the building are prohibited from forming plastic hinges. This is termed as ‘Strong Column Weak Beam’ design approach. As it can be seen, the plastic hinges will need to rotate more as more deformation of the structure to resist seismic forces will be needed. Larger rotation of the plastic hinges will require a more ductile detailing of the locations of the plastic hinges, such that the hinges can keep on rotating without sudden failure.
Therefore, if a structure is designed for a large Response Modification Factor, R which is capable for large deformation then the designer must ensure proper detailing such that the plastic hinges can have sufficient ductility which can provide for large inelastic rotation demand. In order to maintain stability, the designer also needs to ensure that the columns are capacity protected and prohibited from forming plastic hinges.
This is true not only for structure with large R, but also equally valid for any structure designed with R greater than 1. Although, structures with lower Response Modification Factor, R, may require less detailing as the demand in plastic hinges will be lower than the structures with higher R as can be seen in Figure-2 but seismic design consideration and associated detailing should be considered nonetheless. Although, not discussed here, other important aspects such as P-∆ effect, inter-story drift, etc must also be properly considered during the design.
Therefore, it can be seen that if a structure is located in a seismically active areas where a structure is designed past its elastic limit (i.e. R>1) for a design seismic event, regardless of wind forces greater or lower than the seismic forces, adequate seismic design and detailing consideration must be given to the structure.
1. American Society of Civil Engineers (ASCE), 2010,”Minimum Design Loads for Buildings and Other Structures” ASCE 7-10. Reston, VA: ASCE
2. Federal Emergency Management Agency (FEMA),”NEHRP Recommended Provisions: Design Examples”.